In the operation of a fluid system serving a plurality of work elements, the work elements often demand large volumes of fluid from their associated hydraulic fluid pump. Situations arise where the work elements demand fluid at a rate greater than the capacity of the pump, thus flow-limiting occurs. In such situations, one or more of the work elements, for example, demand more fluid than they are capable of receiving, while another work element requires fluid at a very high pressure in order to continue function under its existing load.
In a series arrangement, the "upstream" work elements receive the needed fluid first, leaving the "downstream" elements to starve. In a parallel arrangement of work elements, the fluid follows the path of least resistance. Therefore, the elements having the lowest load pressures are supplied fluid first, leaving the work elements demanding a higher load pressure with an insufficient fluid flow.
From an operator's perspective, proportional control of the work elements is provided via "manual" controls (i.e., joysticks connected to a valve controlling means) while the pump or pumps are not flow-limited. Once the flow capacity of the pump or pumps is exceeded, however, the hydraulic system reverts to a fixed implement priority such as described above. In this state, controllability of the work elements is severely limited. Attempts by the operator to adjust his inputs correctly to avoid or overcome this state often lead to operator fatigue and poorer production. In addition, automatic functions, such as an auto dig cycle for an excavator, can not be implemented on such a machine. When flow limiting occurs during an automatic function cycle, the machine stalls or incorrectly performs the function.
This problem associated with a plurality of work elements can be solved by implementing a pump or system of pumps having a capacity greater than the total demand capacity ever required by the work elements. However, the resultant pump or system of pumps is prohibitively large, expensive, and inefficient. Additionally, the extra weight causes the vehicle to consume more fuel and be more costly to maintain.
It is, therefore, desirable to provide an apparatus which monitors and controls the system so as to anticipate a flow-limiting condition and automatically reduce fluid delivery rates to said work elements and maintain flow proportional to their individual actual demand.
The present invention is directed to overcoming one or more of the problems as set forth above.